CI: Make sure all python requirements are installed in Cygwin CI.

For some reason wheel wasn't installed properly.

.github/workflows/test-cygwin.yml

@@ -21,18 +21,24 @@ jobs:
         with:
           platform: x64
           packages: >
-            python38-devel python38-tkinter libfreetype-devel
-            libimagequant-devel libjpeg-devel liblcms2-devel
-            libopenjp2-devel libraqm-devel libtiff-devel libwebp-devel
-            libxcb-devel python38-olefile python38-pathlib
-            python38-numpy netpbm ImageMagick jpeg xorg-server-extra
-            xorg-server-common xinit python38-setuptools
-            python38-wheel
+            libfreetype-devel libimagequant-devel libjpeg-devel
+            liblcms2-devel libopenjp2-devel libraqm-devel
+            libtiff-devel libwebp-devel libxcb-devel netpbm
+            ImageMagick jpeg xorg-server-extra xorg-server-common
+            xinit python38-setuptools python38-devel python38-tkinter
+            python38-wheel python38-pip python38-olefile
+            python38-numpy python38-pytest python38-sphinx
+            python38-packaging python38-cffi
           install-dir: 'C:\tools\cygwin'
       - name: Clean up path
         uses: egor-tensin/cleanup-path@v2
         with:
           dirs: 'C:\tools\cygwin\bin;C:\tools\cygwin\lib\lapack'
+      - name: Ensure python dependencies installed
+        shell: 'C:\tools\cygwin\bin\dash.exe {0}'
+        run: |
+          /usr/bin/python3.8 -m pip install pip wheel setuptools
+          /usr/bin/python3.8 -m pip install -r requirements.txt
       - name: Build Pillow
         shell: 'C:\tools\cygwin\bin\dash.exe {0}'
         run: |